lung volume measurement andventilation distribution during

Lung Volume Measurement and Ventilation Distribution During InvasiveMechanical Ventilation

Thomas Piraino

Introduction

Lung Volume Measurements During Mechanical Ventilation

Computed Tomography

Inert Gas Dilution (Helium-Dilution Method)

Nitrogen Washin/Washout

Electrical Impedance Tomography to Estimate EELV

Why Measure Lung Volumes During Invasive Ventilation?

Do We Need to Measure Lung Strain?

Ventilation Distribution

Lung volume measurement performed during invasive mechanical ventilation can be used to

determine functional residual capacity, changes in end-expiratory lung volume with the applica-

tion of PEEP, and lung strain. However, many bedside measurements provide useful information

without the use of specialized equipment. Ventilation distribution through the lung has tradition-

ally been assessed with computed tomography, but more recently electrical impedance tomogra-

phy has brought the ability to monitor this at the bedside, and without exposure to radiation.

This review will describe techniques to measure lung volumes in the ICU and the relationship

between lung strain, stress, and other measurements. This review will also discuss monitoring

ventilation distribution at the bedside and the clinical assessment of regional compliance that

this technology provides. Key words: lung volume measurement; electrical impedance tomography;mechanical ventilation. [Respir Care 2020;65(6):760–771. © 2020 Daedalus Enterprises]

Introduction

The measurement of lung volume is required to diagnose

a variety of conditions in out-patient clinics, but its utility

during mechanical ventilation is less common and is not

considered routine practice. Lung volume measurements

can be used to determine the safety of mechanical ventila-

tion, but simpler methods exist (eg, plateau pressure).

Ventilation distribution is traditionally determined with

computed tomography (CT) and is not considered a routine

measurement during mechanical ventilation. Electrical im-

pedance tomography (EIT) is a noninvasive and radiation-

free imaging technique that uses a minimum of 16 leads

positioned around the thorax. EIT provides breath-by-

breath dynamic imaging of ventilation distribution through

measuring impedance changes across lung regions. The

availability of EIT at the bedside has generated a growing

interest in the information provided and how it can used to

adjust ventilator settings.Mr Piraino is affiliated with St. Michael’s Hospital, Toronto, Ontario,

Canada.

Mr Piraino presented a version of this paper at the 58th RESPIRATORY

CARE Journal Conference, Monitoring Respiratory Function in Adult

Acute Care, held June 14–15, 2018, in St Petersburg, Florida.

Mr Piraino has disclosed relationships with Drager, Philips, and Fisher &

Paykel.

Correspondence: Thomas Piraino RRT FCSRT FAARC, St. Michael’s

Hospital, 30 Bond Street, Toronto, Ontario M5B 1W8, Canada.

E-mail: [email protected].

DOI: 10.4187/respcare.07467

760 RESPIRATORY CARE � JUNE 2020 VOL 65 NO 6

mailto:[email protected]

Lung Volume Measurements During Mechanical

Ventilation

Lung volume measurements, such as inspiratory capacity,

vital capacity, or even expiratory reserve volume, can be

measured using simple spirometry measurements, whereas

measurements of total lung capacity and functional resid-

ual capacity (FRC) require more sophisticated methods.

Although these methods are routinely performed in pul-

monary function labs to diagnose various lung diseases,

their use in the ICU is far less common.

The gold standard for measuring lung volumes in out-

patient clinics is by using a device commonly referred to as

a body box. This method employs Boyle’s law to determine

changes in volume relative to pressure. For obvious reasons

(ie, size and shape), the body box is not an option for meas-

uring lung volumes in patients undergoing invasive me-

chanical ventilation. Although there exists no true gold

standard for measuring lung volumes in invasively venti-

lated patients, CT has become the standard comparison

because it is not affected by lung pathology and it likely

reflects the most accurate measure of end-expiratory lung

volume (EELV). However, this technique typically requires

patient transport and exposure to radiation. Bedside meth-

ods for measuring absolute lung volume include inert gas

(eg, helium) dilution and the nitrogen washin/washout tech-

nique. Although these methods are limited by lung pathol-

ogy (ie, only areas with ventilation distribution and gas

exchange will be measured), they can be done at the bed-

side without patient transport or radiation exposure.

Computed Tomography

CT has become a standard for measuring lung volume in

critically ill patients. However, it is a significant source of

radiation to patients as it uses beams of x-ray electrodes

aimed at detectors in a circular arrangement that the patient

is passed through. The device rotates around the patient to

create a slice along the transverse plane, and an image is

generated based on the attenuation of the x-ray beam

caused by differences in tissue density. The result is a 2-

dimensional axial image composed of multiple pixels. The

movement of the patient through the CT scanner allows for

a 3-dimensional pixel called a voxel to be rendered. The

degree of attenuation caused by air or tissue is given a value

called a Hounsfield unit, which represents the density of the

tissue within an arbitrary scale; the range of this scale is

from �1,000 to 1,000, with water being 0 and air being

�1,000.1 Today’s CT scanners can image an entire thorax

in a single breath, and computer software can reconstruct

the image to provide an accurate volume measurement for

both gas and tissue. To calculate lung volume, the follow-

ing equation is used:

gas volume ¼ CTnumber�1; 000

� �� voxel volume:

Although CT can accurately measure lung volume

related to inflation, it does not determine the presence of air

compared to ventilated regions. Therefore, the presence of

lung volume does not automatically imply efficiency of gas

exchange, particularly in patients with hyperinflation due to

severe airway disease or COPD.2 The more common use of

CT in the literature related to invasive mechanical ventila-

tion is in the area of lung recruitment, lung stress, and lung

strain. However, even if the need to transport patients could

be eliminated by the use of a portable CT scanner, there

remains the issue of radiation exposure. This has resulted in

the evaluation of other noninvasive, nonradiologic methods

for measuring lung volume at the bedside.

Inert Gas Dilution (Helium-Dilution Method)

Lung volume measurements using inert gas dilution is

based on the equilibration between the lungs and a known

volume of the inert gas. The inert gas dilution method that

has been used for many years in spontaneously breathing

patients for lung volume assessment is the helium-dilution

method. The common helium-dilution technique used in

spontaneously breathing patients has been modified for use

during invasive mechanical ventilation. In 2004, Patroniti

et al3 conducted a study in 21 subjects with mild-to-moder-

ate ARDS to test the accuracy and precision of a simplified

bedside helium-dilution method compared to CT. The sub-

jects were sedated, paralyzed, and placed on a CPAP level

equal to the clinically set PEEP level for the duration of the

CT scan to keep EELV constant. The EELV determined

from the CT (EELVCT) was calculated using the equation

discussed in the previous section. After restarting ventila-

tion for a 5-min stabilization period, an end-expiratory

pause was performed, the endotracheal tube was clamped,

and the subject was disconnected from the ventilator. A bag

with 1.5 L of gas containing 13.44% helium in oxygen was

connected, the clamp was released, and 10 breaths were

delivered with the bag to dilute the bag mixture with the

gas from the subject’s lungs. The bag was then clamped

and separated from the patient, and the ventilator was con-

nected and ventilation resumed. The gas was analyzed with

a calibrated heliometer, and EELV determined with this

method (EELVHe) was calculated with the following

equation:

EELVHe ¼ Vb � Ci

Cf

� �� Vb

where Vb is the initial volume of gas in the bag, Ci is the

initial concentration of helium, and Cf is the final concen-

tration of helium.

LUNG VOLUME MEASUREMENT DURING INVASIVE MECHANICAL VENTILATION

RESPIRATORY CARE � JUNE 2020 VOL 65 NO 6 761

After another 5 min of ventilation (ie, a stabilization pe-

riod), the helium dilution measurement was performed

again to test the precision of multiple measurements. There

was a linear correlation between EELVCT and EELVHe:

EELVHe ¼ 208 + 0.858 � EELVCT (r ¼ 0.941, P < .001).

However, EELVCT was overestimated at low lung volumes

and underestimated at high volumes. The bias between

these methods was 32.5 6 202.8 mL (4.75 6 15.9%) with

a 95% CI limit of agreement of �373 and 438 mL and an

absolute percent difference of 14.66 10.1%. The large lim-

its of agreement could be explained by a number of factors,

including the loss of PEEP when subjects were connected

to the helium-dilution bag, and also because EELVHe repre-

sents ventilated regions only, whereas EELVCT would rep-

resent regions that are aerated but may not be ventilated

(and contributing to gas exchange). Despite the large limits

of agreements, the authors suggest that this is within clini-

cally acceptable limits (Table 1).3

The assessment of multiple EELVHe measurements had

a bias of �24 6 83 mL (�2.1 6 9.6%), and the 95% CI

limits of agreement between the measurements was �191

mL and 141 mL, with an absolute difference of 6.3 67.4%. This supports the reproducibility of this type of he-

lium-dilution method. Finally, the authors acknowledge the

fact that patients require disconnection from the ventilator

and may experience a significant loss of PEEP; however,

they stated there were no significant effects on respiratory

or hemodynamic stability.3

NitrogenWashin/Washout

Another technique for measuring lung volumes at the

bedside is the nitrogen (N2) washin/washout method. This

technique is based on the principle that the volume of N2 in

the lung is based on the fraction of N2 multiplied by the

EELV. Because FiO2is inversely related to the fraction of

N2 in a volume of alveolar gas, the relationship to EELV

can be written as EELV ¼ DN2/DFIO2, where DN2 is the

change in N2 content (in mL) after a stabilization period

of DFIO2. The line of ventilators from GE Healthcare

(Madison, Wisconsin) allow bedside measurement of lung

volume using the N2 washin/washout method. However,

rather than measuring the concentration of N2, the ventila-

tor estimates it by measuring the concentrations of CO2 and

O2. Chiumello et al2 compared the N2 washin/washout

technique to both the helium-dilution method and the CT

method in 30 subjects admitted to the ICU; they did not

exclusively enroll patients with ARDS as in the previously

mentioned helium-dilution study. The authors reported that

the N2 washin/washout technique also had excellent corre-

lation with the CT method (r2 ¼ 0.89) with a bias of 94 6143 mL (15 6 18%, P < .001) and limits of agreement of

�50 mL to �236 mL). The average overestimation was

also acceptable (less than the manufacturer’s accuracy limit

of 20%). Table 1 provides a comparison of methods and

their limits of agreement with lung volumes measured with

CT. The authors also compared repeated measurements of

the N2 washin/washout, which exhibited excellent reprodu-

cibility between 2 measurements of 48 6 165 mL, which

was not statistically different than having no difference

between measurements (P¼ .12).

Similar to the previously mentioned study on the helium-

dilution method,3 the authors also reported excellent corre-

lation between EELV with the helium-dilution method and

EELV measured with CT (r2 ¼ 0.91). There was a negative

bias of �136 6 133 mL (16 6 13%) with limits of agree-

ment of �3 mL to �269 mL. A possible explanation for

the different results from the previous helium-dilution

study3 is the inclusion of only subjects with ARDS.

Patients with ARDS typically have lower lung volumes,

as opposed to patients with normal lung compliance who

have higher lung volumes. Thus the 10 breaths used with

this method may not have been sufficient to allow suffi-

cient gas equilibration. Additionally, the differences

between N2 washin/washout and helium-dilution values

could also be due to the number of breaths used. The

N2 washin/washout technique uses 20 breaths for equili-

bration compared to the 10 breaths used in the helium-

dilution method.

Electrical Impedance Tomography to Estimate EELV

In addition to measuring ventilation distribution through

the lung, which will be discussed later, EIT devices also pro-

vide additional tools for measuring changes in end-expira-

tory lung impedance (DEELI), both globally and to selectedregions of interest. Impedance changes during tidal breath-

ing (tidal variation) represent changes in tidal volume, and

Table 1. Methods of lung volume measurement compared to computed tomography

Method Bias (% Difference) Limits of Agreement Considerations

Helium dilutiona 32.5 6 202.8 mL (4.8) �373 to 438 mL ARDS patients only

Helium dilutionb �136 6 133 mL (16) �3 to �269 mL Excluded COPD patients

Nitrogen washin/washoutb 94 6 143 mL (15) �50 to �236 mL Excluded COPD patients

a From Reference 3.b From Reference 2.



this tidal variation value, measured in arbitrary units called

DZ, is used as a reference value to estimate change in

EELV. By comparing DEELI to the height of a reference

tidal breath, the volume of change is inferred (Fig. 1).

A number of studies have compared changes in EELV

estimated by DEELI to helium dilution and N2 washin/wash-

out, but not to CT.4-7 All of these studies found good or

excellent correlation between EIT estimates of EELV and

other methods, but these studies have not considered the lim-

its of agreement to be clinically acceptable for estimating

actual volume change.

A differentiating factor between DEELI measurements

and other bedside methods is that EIT captures a field or

region around the area where the EIT belt is placed.

Although this region is larger than the belt width, it may not

represent all regions being ventilated with the N2 washin/

washout or helium-dilution techniques. However, DEELI isnot affected by ventilation changes and can measure DEELIin lung regions that may not be ventilating well, making it

potentially similar to CT scans. A proper study comparing

EELV changes estimated by EIT to CT would be useful

because it is unknown whether the values are closer to those

found with a CT scan than the other methods.

The DEELI has been used as a method to set optimal

PEEP in patients with ARDS. Eronia et al8 enrolled 16

patients with hypoxemic respiratory failure of noncar-

diogenic origin into a study to assess the use of EIT to

set a PEEP level that maintains alveolar recruitment af-

ter a recruitment maneuver. Overall, they found the

method to be safe despite noting that it required the use

of higher PEEP. The method also resulted in lower driv-

ing pressure and an increase in oxygenation. In addition,

although it was feasible in most subjects, there were 2

subjects with unstable EELI trends, which precluded

any analysis.

There are limitations to measuring DEELI with EIT. Thestability of the EELI trend is largely affected by external

pressure and aggressive fluid balance shifts. Certain ICU

bed mattresses with pulsating pressure can greatly influence

the stability of the EELI trend over time (Fig. 2). Fluid re-

moval with either dialysis or simply a bolus of furosemide

can also affect impedance measurements, causing an

increase in EELI that suggests an improvement in EELV

without any changes in respiratory system mechanics or

distribution of ventilation. Administering a fluid bolus can

also cause a downward shift in EELI that suggests a loss in

Fig. 1. End-expiratory lung volume (EELV) can be estimated by using the reference tidal volume (white arrow) and multiplying it by the globalchange in end-expiratory lung impedance (DEELI) of 1.39. Additionally, DEELI of each region can be used to estimate the change in EELV for

each region of interest. Although the change volume correlates with other methods of lung volume measurements, the limits of agreement arenot considered to be clinically acceptable.

Global TV global C1

Fig. 2. The effect of a pulsating bed mattress on the end-expiratory lung impedance trend.



EELV, again without any changes in respiratory system

mechanics or distribution of ventilation.9

WhyMeasure Lung Volumes During Invasive

Ventilation?

An increase in PEEP will always increase EELV, and re-

spiratory-system compliance can predict the amount of vol-

ume it would increase. For example, if your compliance is

50 mL/cm H2O, an increase of 10 cm H2O would increase

EELV by 500 mL (50 mL� 10 cm H2O). If EELV is meas-

ured and found to increase more than expected, increasing

PEEP has recruited alveolar units. Patients who exhibit

recruitment when PEEP is increased are considered PEEP

responders, and higher PEEP is likely more beneficial.

However, PEEP and EELV contribute to lung strain.

Strain refers to the deformation, or change, in the shape

of a structure compared to its resting condition.10 Lung

strain is defined as the change in volume relative to FRC,

and lung volume measurements are used to measure FRC

in mechanically ventilated patients.10 For the calculation of

lung strain, values of FRC are obtained at zero end-expira-

tory pressure: lung strain ¼ DV/FRC, where DV refers to

the change in volume during inspiration. The concept of

lung strain becomes complicated and less intuitive with

the application of PEEP. As already stated, the application

of PEEP always increases EELV. However, some con-

sider the impact of PEEP-induced EELV to be in the nu-

merator of the strain equation,10 which would increase

strain: lung strain ¼ (DV + EELV)/FRC, whereas other

authors appear to group EELV (from PEEP) and FRC in

the denominator: lung strain ¼ DV/(FRC + EELV).7,11

The difference in concepts are likely due to the fact that

both are possible, but both rely heavily on the recruitment

potential of the lung. A ventilated patient with significant

atelectasis would have low FRC but may be highly recruit-

able, hence an increase in PEEP (increasing EELV) would

likely improve the resting state and decrease strain (EELV

in the denominator). A ventilated patient with consolidated

lung regions or with pleural effusions may not be overly

recruitable and the application of PEEP (increasing EELV)

would likely increase strain (EELV in the numerator). If

we consider recruitable volume (Vrec) when increasing

PEEP, lung strain is likely a compromise:

Lung strain ¼ DV þ ðEELV�VrecÞFRC þ Vrec

:

Lung volume measurements are feasible and safe, but

whether they add clinical value or guide clinicians to make

better clinical decisions to manage their ventilated patients

compared to other more common measurements is still

unknown.

DoWe Need to Measure Lung Strain?

Lung stress is considered to be the internal distribution

of counterforce in a given area that reacts to an external

load.10 In clinical terms, lung stress refers to the distending

pressure within the lung, and the counterforce (external

load) is the chest wall. Therefore, the stress on the lung is

the transpulmonary pressure. Lung stress is directly related

to lung strain: stress ¼ k � strain, where k is the specific

lung elastance (�13.5 cm H2O in humans).10 Although

stress can be estimated with transpulmonary pressure meas-

urements (with esophageal balloon), these measurements

are still not common practice in the majority of ICUs.12

Simple bedside assessments can assist clinicians in deter-

mining whether they are potentially increasing stress or

strain to the patient’s lungs. For example, both lung stress

and strain have a linear relationship with plateau pres-

sure.10,13 Maintaining a plateau pressure < 25 cm H2O in

most patients (< 30 cm H2O in patients with ARDS) would

limit lung strain to < 2 (considered detrimental) and lung

stress to 22–24 cm H2O (considered the upper limit of

stress).10

A bedside maneuver for assessing recruitability has also

been described.14 Just as lung volume measurements to

measure EELV after a PEEP change can suggest recruit-

ment if the DEELV is greater than expected, this simplified

maneuver involves decreasing PEEP by 10 cm H2O in a

single-breath and observing the exhaled volume during the

transition from higher to lower PEEP. The expected loss

(exhaled tidal volume + predicted loss from PEEP change

based on respiratory-system compliance at low PEEP) is

subtracted from the exhaled volume. The excess loss is

used to determine the recruitment-to-inflation ratio (Fig.

3).15 Validation of the recruitment-to-inflation ratio using

lung recruitment measurements (multiple P-V curve

method) was published recently.15 An online calculator is

also available at (https://crec.coemv.ca, Accessed April 3,2020).In patients with ARDS, oxygenation by itself may not

be a good indicator of patient outcome.16 However, in a

secondary analysis of the Lung Open Ventilation Study

(LOVS), a positive oxygenation response to PEEP change

was associated with a lower probability of death.17,18 The

authors noted that an improvement in oxygenation after

PEEP change was associated with reduced mortality in a

multivariate logistic regression analysis (adjusted odds ra-

tio 0.80, 95% CI 0.72–0.89, for every 25 mm Hg increase

in PaO2/FIO2

).18

Although driving pressure (plateau pressure � PEEP) >15 cm H2O has been shown to be associated with higher

mortality in subjects with ARDS,19 the best limit has not

been tested in a prospective randomized trial.20 Despite the

lack of prospective data, making changes to ventilator set-

tings that maintain or even lower driving pressure may not



https://crec.coemv.ca

have the same impact on lung stress or strain compared

to changes that result in a significant increase in driv-

ing pressure (an increase is likely more injurious).

However, the lowest driving pressure, or best compli-

ance, does not always indicate that ventilation is the

safest.21 In fact, the best compliance may not reflect the

optimal balance between overdistention and collapse

assessed with EIT.22,23

More complex concepts have recently been introduced

regarding the amount of mechanical power applied to the

lung during positive-pressure ventilation.24 The total energy

of mechanical ventilation is composed of multiple compo-

nents including tidal volume, PEEP, driving pressure (ie,

compliance), flow, and breathing frequency. Considering

that PEEP, plateau pressure, and tidal volume should be set

individually, when these elements are excessive, or when

the breathing rate is considerably high, all elements factor

into potential for injury and should be minimized whenever

possible. However, when faced with the dilemma of reduc-

ing the pressure applied to the lung or respiratory rate, pres-

sures applied to the lung (ie, plateau pressure or driving

pressure) have consistently been associated with mortality

and should take priority. Consider permissive hypercapnia

to help with lowering breathing frequency.

In summary, limiting plateau pressure, assessing recruit-

ability using a single-breath decremental PEEP maneuver

(ie, recruitment-to-inflation ratio), observing the oxygen-

ation response when increasing PEEP, monitoring driving

pressure, and allowing permissive hypercapnia are simple

strategies that can assist in clinical decisions for mechani-

cally ventilated patients, particularly those with ARDS.

However, lung strain and transpulmonary pressure meas-

urements (lung stress) may be helpful in the personalization

of mechanical ventilation when ventilating a patient at or

above the limits of recommended pressures is required yet

may be safe. Patients with very high baseline pleural pres-

sure or a stiff chest wall may benefit from measurements of

lung stress and strain to confirm the safety of ventilator

settings.

Ventilation Distribution

Ventilation distribution is traditionally determined with

CT. In 1995, Gattinoni and colleagues25 described the

Pres

sure

PEEP 15 cm H2O

Set VT

VT released Predicted exhaled VT

Exhaled VT recruitment

Exhaled VT

Plateau

PEEP 5 cm H2O�10 cm H2O

Flow

Volu

me

Time

Fig. 3. The recruitment-to-inflation ratio is a ratio between compliance of recruited lung (on high PEEP) compared to respiratory-system compli-ance (on low PEEP). It uses continuous mandatory ventilation (ie, a passive patient) and a 10 cm H2O drop in PEEP (from high to low).

Compliance of the recruited lung (due to the higher PEEP) is determined by first subtracting the exhaled volume at high PEEP and the predictedloss of volume (predicted based on respiratory-system compliance) from the exhaled volume released during the PEEP drop, then dividing the

remaining volume (ie, recruitment) by the drop in pressure (10 cm H2O). Respiratory system compliance is determined by the set tidal volume(VT) divided by plateau pressure – PEEP at the lower PEEP level. The compliance of the recruited lung is divided by the compliance of the respi-ratory system to determine the ratio. A ratio of$ 0.5 indicates recruitment potential (the higher the value above 0.5, the greater the recruitment

potential), suggesting higher PEEP recruits lung units. The presence of airway closure should first be assessed before performing the maneu-ver. Video demonstrations of both maneuvers and an online calculator are available at (https://crec.coemv.ca, Accessed April 3, 2020).



https://crec.coemv.ca

response to PEEP in 8 subjects with ARDS in terms of re-

gional distribution, particularly in the ventral, middle, and

dorsal lung regions. The authors described the importance

of plateau pressure as the opening pressure of the lung, and

PEEP as the pressure that can maintain opened and recruited

alveolar units. However, if the plateau pressure reached

does not recruit dorsal regions of the lung, the result is over-

distention (and poor perfusion) to the ventral regions of the

lungs, with a lack of distribution to the dorsal lung regions.

When the lung is open and PEEP maintains recruitment,

tidal volume distribution during inspiration is maintained

closer to a 1:1 ratio between the ventral and dorsal lung

regions when considering a caudal-cranial view. Until

recently, the ability to view distribution changes throughout

the lung at the bedside was not considered possible.

The use of EIT monitoring has made the assessment of

ventilation distribution at the bedside a reality. Considering

that lung recruitability plays an important role in the appro-

priate setting of PEEP and the resulting lung stress and

strain, the ability to visually assess ventilation distribution

changes, as they happen, at the bedside is a giant step for-

ward in bedside monitoring. Similarities between ventilation

distribution and lung CT have also been assessed. In 2004,

Victorino et al26 performed a validation study of regional

lung ventilation measured with EIT compared to CT. They

found that changes in air content measured with CT highly

predicted regional changes in impedance (r2 ¼ .93), which

means that impedance changes occurring during ventilation

measured with EIT explain ventilation changes observed in

CT (93% of its variance). Right-to-left differences in venti-

lation distribution show a bias of 0% and limits of agree-

ment of �10% to 10% (P ¼ .31). Although CT and EIT

detected similar imbalances between the ventral and dorsal

distribution, with good matching within each case, EIT

showed a marginally larger imbalance between the ventral

and dorsal distribution (P¼ .04).

In addition to regional ventilation changes, regional

compliance changes can also be determined with EIT.

Considering that tidal variation represents tidal volume

delivery, compliance changes can be calculated according

to changes in tidal variation and the applied ventilation

pressure. Instead of mL/cm H2O, EIT measures percent

changes in DZ/cm H2O. EIT software is now available on

devices to compare changes in regional compliance during

PEEP titration, including an approach often referred to as

the Costa approach, named after the first author of the study

that described this method.22 This regional compliance

analysis compares compliance changes that occur in the

transition from a higher level of PEEP to a lower level of

PEEP and displays areas with poor compliance at high and

low PEEP levels. Compliance loss at high levels of PEEP

suggest areas of overdistention, and compliance loss at low

levels of PEEP suggest collapse (Fig. 4). One limitation is

that each maximum and minimum level of PEEP represents

the point of zero change; this does not actually mean there

is no collapse or overdistention at the upper and lower lim-

its of the PEEP trial, rather it is simply the reference point

for change. Coincidentally, the balance between overdisten-

tion and collapse determined by EIT does not imply best re-

spiratory-system compliance.22,23

Although a spontaneous breathing trial and ventilator

liberation remains the goal for every invasively venti-

lated patient, the level of spontaneous effort is important.

Very low inspiratory effort can result in prolonged time

on the ventilator, as does excessive inspiratory effort.27

Spontaneous breathing is also a controversial topic for

patients with ARDS.28 Additionally, excessive inspiratory

effort has been described as a potential mechanism for self-

inflicted lung injury.28,29 One of the potential mechanisms

driving the potential for injury is the regional strain that can

occur in the lung due to high inspiratory effort.30 In an ani-

mal model of lung injury, Yoshida et al31 demonstrated that,

despite controlling tidal volume (ie, using volume-controlled

ventilation), inspiratory effort can cause a pendelluft effect

where volume is pulled from the ventral regions into the

dorsal regions, which results in excess strain in the dorsal

region. This can be seen visually by monitoring the percent-

age of tidal volume delivered to the region.

Fig. 4. The Costa approach refers to the plotting of regional compliance after performing a stepwise PEEP trial. Loss of regional compliance athigh levels of PEEP is plotted against compliance loss at low levels of PEEP.



The regions of interest monitored with EIT are normally

customizable, but common configurations are available on

EIT devices. These configurations include regions such as 4

even horizontal slices (layers), quadrants, left and right sepa-

ration, and ventral and dorsal separation. The distribution of

tidal variation is normally given as a percentage of the

global tidal variation (representing tidal volume). However,

the correlation between tidal volume and tidal variation

depend upon proper belt placement.32 Ideally, the EIT belt

should be placed between the fourth and fifth intercostal

space; in female patients, the belt is normally placed above

the breast tissue. With proper belt placement, one of the

simplest approaches to monitoring distribution of ventilation

is the ventral/dorsal configuration. During spontaneous

breathing, excessive support can lower patient drive and the

resulting effect can be poor distribution to the dorsal regions

compared to ventral regions. In a paper by Mauri and col-

leagues,33 as pressure support decreased, patient effort

increased, resulting in an increase in dorsal region ventila-

tion. Proper monitoring of patient effort is important when

implementing such a strategy to avoid excessive patient

effort that could lead to prolonged mechanical ventilation.34

Monitoring of the ventral/dorsal distribution can provide

information regarding the need to attempt an increase in

PEEP, but it can also serve as an incentive to decrease

PEEP.35 When the dorsal fraction of ventilation is low (ie,

< 50%), increasing PEEP should be attempted and moni-

tored for improved distribution and regional compliance

changes. Patients who respond well to increases in PEEP

should exhibit a shift from ventral to dorsal regions (Fig.

5A). When patients with severe ARDS fail to show an ac-

ceptable increase in the dorsal fraction of ventilation (and

oxygenation), EIT can be used to monitor the response to

prone positioning (Fig. 5B). When PEEP is excessive,

ventilation distribution is reduced in the ventral regions and

becomes predominant in the dorsal regions (dorsal fraction

of ventilation > 50%); when this occurs in the supine posi-

tion, PEEP can likely be decreased (Fig. 5C). It is unclear

whether PEEP should be reduced in proned patients with a

dorsal fraction of ventilation > 50%, as this may be due to

collapsed ventral regions (due to gravity), not overdis-

tended regions as it would imply in the supine position.

Although the global ventral/dorsal view is one of the

more simplistic approaches when using EIT, a major limi-

tation is when only 3 quadrants are participating in venti-

lation. In this case, it is better to consider the regional

ventral/dorsal distribution only on the side (left or right)

where the lung is participating most to ventilation. In

Figure 6, quadrants 2 and 4 (Q2 and Q4) are not balanced

at higher PEEP despite the dorsal fraction of ventilation

being 52%. When PEEP is lowered, the dorsal fraction is

now only 40%, but Q2 and Q4 are better balanced (36%

and 35%, respectively).

Whether the clinical use of EIT results in better patient

outcomes has yet to be determined. Clinical experience is

growing, but widespread use has not been established.

Additionally, there is still no consensus on which parame-

ters should be used to adjust ventilator settings such as

PEEP. The ability to monitor the distribution of ventilation

and regional compliance offers information that has never

been available before at the bedside, making EIT a very

exciting and promising bedside monitoring tool for research

and, more importantly, clinical practice.

REFERENCES

1. Campos MA, Diaz AA. The role of computed tomography for the

evaluation of lung disease in alpha-1 antitrypsin deficiency. Chest

2018;153(5):1240-1248.

2. Chiumello D, Cressoni M, Chierichetti M, Tallarini F, Botticelli M,

Berto V, et al. Nitrogen washout/washin, helium dilution and com-

puted tomography in the assessment of end expiratory lung volume.

Crit Care 2008;12(6):R150.

3. Patroniti N, Bellani G, Manfio A, Maggioni E, Giuffrida A, Foti G,

et al. Lung volume in mechanically ventilated patients: measurement

by simplified helium dilution compared to quantitative CT scan.

Intensive Care Med 2004;30(2):282-289.

4. Bikker IG, Leonhardt S, Bakker J, Gommers D. Lung volume calcu-

lated from electrical impedance tomography in ICU patients at differ-

ent PEEP levels. Intensive Care Med 2009;35(8):1362-1367.

28%Q2

44%Q4

47%VFV

52%DFV

PEEP 16 PEEP 10

36%Q2

35%Q4

60%VFV

40%DFV

Fig. 6. When only 3 quadrants are being ventilated, the global dorsalfraction of ventilation (DFV) should not be used. In this example, the

left lung (right side of the image) should be balanced between ven-tral and dorsal regions. VFV¼ ventral fraction of ventilation.

68%VFV

32%DFV

A

B

C

70%VFV

30%DFV

62%VFV

38%DFV

36%VFV

64%DFV

36%VFV

64%DFV

47%VFV

53%DFV

52%VFV

48%DFV

Fig. 5. A: Dorsal fraction of ventilation (DFV) increasing in a PEEP re-sponder. B: DFV in a nonresponder before and after PEEP titrationand after being placed in the prone position. C: DFV when PEEP is

excessive and after reduction. There is minimal response to PEEPwith significant loss of regional compliance in the ventral regions.

VFV¼ ventral fraction of ventilation.



5. Hinz J, Hahn G, Neumann P, Sydow M, Mohrenweiser P, Hellige G,

et al. End-expiratory lung impedance change enables bedside monitor-

ing of end-expiratory lung volume change. Intensive Care Med 2003;

29(1):37-43.

6. Karsten J, Meier T, Iblher P, Schindler A, Paarmann H, Heinze H. The

suitability of EIT to estimate EELV in a clinical trial compared to oxy-

gen wash-in/wash-out technique. Biomed Tech (Berl) 2014;59(1):

59-64.

7. Mauri T, Eronia N, Turrini C, Battistini M, Grasselli G, Rona R, et al.

Bedside assessment of the effects of positive end-expiratory pressure

on lung inflation and recruitment by the helium dilution technique and

electrical impedance tomography. Intensive Care Med 2016;42(10):

1576-1587.

8. Eronia N, Mauri T, Maffezzini E, Gatti S, Bronco A, Alban L, et al.

Bedside selection of positive end-expiratory pressure by electrical im-

pedance tomography in hypoxemic patients: a feasibility study. Ann

Intensive Care 2017;7(1):76.

9. Becher T, Wendler A, Eimer C, Weiler N, Frerichs I. Changes in elec-

trical impedance tomography findings of ICU patients during rapid

infusion of a fluid bolus: a prospective observational study. Am J

Respir Crit Care Med 2019;199(12):1572-1575.

10. Chiumello D, Carlesso E, Cadringher P, Caironi P, Valenza F, Polli F,

et al. Lung stress and strain during mechanical ventilation for acute re-

spiratory distress syndrome. Am J Respir Crit Care Med 2008;178(4):

346-355.

11. Gonzalez-Lopez A, Garcıa-Prieto E, Batalla-Solıs E, Amado-

Rodrıguez L, Avello N, Blanch L, Albaiceta GM. Lung strain and bio-

logical response in mechanically ventilated patients. Intensive Care

Med 2012;38(2):240-247.

12. Bellani G, Laffey JG, Pham T, Fan E, Brochard L, Esteban A, et al.

Epidemiology, patterns of care, and mortality for patients with acute

respiratory distress syndrome in intensive care units in 50 countries.

JAMA 2016;315(8):788-800.

13. Loring SH, O’Donnell CR, Behazin N, Malhotra A, Sarge T, Ritz R,

et al. Esophageal pressures in acute lung injury: do they represent arti-

fact or useful information about transpulmonary pressure, chest wall

mechanics, and lung stress? J Appl Physiol (1985) 2010;108(3):515-

522.

14. Chen L, Chen GQ, Shore K, Shklar O, Martins C, Devenyi B, et al.

Implementing a bedside assessment of respiratory mechanics in

patients with acute respiratory distress syndrome. Crit Care 2017;21

(1):84.

15. Chen L, Del Sorbo L, Grieco DL, Junhasavasdikul D, Rittayamai N,

Soliman I, et al. Potential for lung recruitment estimated by the recruit-

ment-to-inflation ratio in acute respiratory distress syndrome. Am J

Respir Crit Care Med 2020;201(2):178-187.

16. Acute Respiratory Distress Syndrome Network, Brower RG, Matthay

MA, Morris A, Schoenfeld D, Thompson BT, Wheeler A. Ventilation

with lower tidal volumes as compared with traditional tidal volumes

for acute lung injury and the acute respiratory distress syndrome. N

Engl J Med 2000;342(18):1301-1308.

17. Meade MO, Cook DJ, Guyatt GH, Slutsky AS, Arabi YM, Cooper DJ,

et al. Ventilation strategy using low tidal volumes, recruitment maneu-

vers, and high positive end-expiratory pressure for acute lung injury

and acute respiratory distress syndrome. JAMA 2008;299(6):637-645.

18. Goligher EC, Kavanagh BP, Rubenfeld GD, Adhikari NK, Pinto R,

Fan E, et al. Oxygenation response to positive end-expiratory pressure

predicts mortality in acute respiratory distress syndrome: a secondary

analysis of the LOVS and ExPress trials. Am J Respir Crit Care Med

2014;190(1):70-76.

19. Amato MB, Meade MO, Slutsky AS, Brochard L, Costa EL,

Schoenfeld DA, et al. Driving pressure and survival in the acute respi-

ratory distress syndrome. N Engl J Med 2015;372(8):747-755.

20. Aoyama H, Yamada Y, Fan E. The future of driving pressure: a pri-

mary goal for mechanical ventilation? J Intensive Care 2018;6(1):64.

21. Writing Group For The Alveolar Recruitment For Acute Respiratory

Distress Syndrome Trial Investigators, Cavalcanti AB, Suzumura EA,

Laranjeira LN, Paisani DM, Damiani LP, et al. Effect of lung recruit-

ment and titrated positive end-expiratory pressure (PEEP) vs low peep

on mortality in patients with acute respiratory distress syndrome: a

randomized clinical trial. JAMA 2017;318(14):1335-1345.

22. Costa ELV, Borges JB, Melo A, Suarez-Sipmann F, Toufen C, Bohm

SH, et al. Bedside estimation of recruitable alveolar collapse and

hyperdistension by electrical impedance tomography. Intensive Care

Med 2012;1:165-170.

23. Franchineau G, Brechot N, Lebreton G, Hekimian G, Nieszkowska A,

Trouillet JL, et al. Bedside contribution of electrical impedance to-

mography to setting positive end-expiratory pressure for extracor-

poreal membrane oxygenation-treated patients with severe acute

respiratory distress syndrome. Am J Respir Crit Care Med 2017;196

(4):447-457.

24. Gattinoni L, Tonetti T, Cressoni M, Cadringher P, Herrmann P,

Moerer O, et al. Ventilator-related causes of lung injury: the mechani-

cal power. Intensive Care Med 2016;42(10):1567-1575.

25. Gattinoni L, Pelosi P, Crotti S, Valenza F. Effects of positive end-ex-

piratory pressure on regional distribution of tidal volume and recruit-

ment in adult respiratory distress syndrome. Am J Respir Crit Care

Med 1995;151(6):1807-1814.

26. Victorino JA, Borges JB, Okamoto VN, Matos GF, Tucci MR,

Caramez MP, et al. Imbalances in regional lung ventilation: a valida-

tion study on electrical impedance tomography. Am J Respir Crit Care

Med 2004;169(7):791-800.

27. Goligher EC, Fan E, Herridge MS, Murray A, Vorona S, Brace D,

et al. Evolution of diaphragm thickness during mechanical ventilation:

impact of inspiratory effort. Am J Respir Crit Care Med 2015;192

(9):1080-1088.

28. Yoshida T, Amato MBP, Kavanagh BP, Fujino Y. Impact of spontane-

ous breathing during mechanical ventilation in acute respiratory dis-

tress syndrome. Curr Opin Crit Care 2019

29. Brochard L, Slutsky A, Pesenti A. Mechanical ventilation to minimize

progression of lung injury in acute respiratory failure. Am J Respir

Crit Care Med 2017;195(4):438-442.

30. Yoshida T, Torsani V, Gomes S, De Santis RR, Beraldo MA, Costa

EL, et al. Spontaneous effort causes occult pendelluft during mechani-

cal ventilation. Am J Respir Crit Care Med 2013;188(12):1420-1427.

31. Yoshida T, Nakahashi S, Nakamura MAM, Koyama Y, Roldan R,

Torsani V, et al. Volume-controlled ventilation does not prevent injuri-

ous inflation during spontaneous effort. Am J Respir Crit Care Med

2017;196(5):590-601.

32. Karsten J, Stueber T, Voigt N, Teschner E, Heinze H. Influence of dif-

ferent electrode belt positions on electrical impedance tomography

imaging of regional ventilation: a prospective observational study. Crit

Care 2016;20:3.

33. Mauri T, Bellani G, Confalonieri A, Tagliabue P, Turella M,

Coppadoro A, et al. Topographic distribution of tidal ventilation in

acute respiratory distress syndrome: effects of positive end-expiratory

pressure and pressure support. Crit Care Med 2013;41(7):1664-1673.

34. Goligher EC, Dres M, Fan E, Rubenfeld GD, Scales DC, Herridge

MS, et al. Mechanical ventilation-induced diaphragm atrophy strongly

impacts clinical outcomes. Am J Respir Crit Care Med 2018;197

(2):204-213.

35. Yoshida T, Piraino T, Lima CAS, Kavanagh BP, Amato MBP,

Brochard L. Regional ventilation displayed by electrical impedance to-

mography as an incentive to decrease PEEP. Am J Respir Crit Care

Med 2019;200(7):933-937.



Discussion

Lamberti: Most of the discussion

was on ARDS. How about in obstruc-

tive airways disease where measure-

ment of end-expiratory lung volume

can be very helpful but it’s difficult. Is

there any evidence in the literature in

regard to the obstructive airways dis-

ease patient who’s ventilated?

Piraino: I did not gravitate towards

non-ARDS patients because I knew I

was talking about distribution of venti-

lation with electrical impedance to-

mography (EIT) as well, and I have

not seen anything published specific

to a COPD population. I imagine it

would be useful to look at EELV

(EELI), but the management of a

patient with very bad obstructive pul-

monary disease involves so much

more than end-expiratory lung vol-

ume. I think the bigger issues are often

making sure you have appropriate ex-

piratory time, minimizing air-trapping,

etc. Trying to estimate EELV wouldn’t

be of much interest, but minimizing it

by controlling respiratory rate or

inspiratory time would have value. It

would be difficult to lower EELV

without reducing respiratory rate.

MacIntyre: Jim [Lamberti], you

raise an interesting point. The question

always comes up in obstructive lung

disease, adding PEEP to facilitate the

triggering process. In other words, to

balance the autoPEEP that’s present in

the lung. The theory is to add enough

PEEP that you balance the autoPEEP

but not so much PEEP that you end up

over-distending lung regions. It seems

to me that EIT might be a very nice

tool, and if you’re doing the right thing

the FRC will not increase. There’s a

potential role there.

Piraino: I agree, I think the distribu-

tion of ventilation may be more in-

formative than the end-expiratory lung

volume, at least with PEEP adjust-

ment because you may not reduce

EELI, but distribution of ventilation

would likely change if triggering

became more effective. But I didn’t

find anything published on the subject.

MacIntyre: I don’t have a lot of ex-

perience with EIT. It uses a single

band of electrodes around the chest.

How much of the lung does this

detect?

Piraino: The band itself is a few

inches, however it captures a field. It’s

not just a single slice. For the measure-

ment of EELV it wouldn’t represent

all the regions where nitrogen wash-in

wash-out is measuring, which is every-

thing being ventilated essentially, but

the band itself does capture anywhere

between 10 and 18 cm of the lung

(I’ve seen both ranges mentioned in

my readings). You can’t specifically

say it’s ‘this lobar’ region, but within

the area. The captured region really is

more than half the height of an average

adult lung.

Blanch: What you have shown in

the patient in prone position is anec-

dotal or usually happens when you are

turning patients with ARDS from

supine to prone?

Piraino: What we’ve seen with EIT

and proning patients is that most of the

patients do exactly what you would

expect them to do, you’d expect to

change the distribution towards the

dorsal regions. Where we’ve used it

more clinically is when we are consid-

ering the need for proning with a

patient. If we increase PEEP and we

see that we are able to redistribute tidal

volume at safe pressures, we may wait

to see what the oxygenation response

is. Having said that, I have seen

patients where we prone and do not

get that response. I gave you some

examples of, ‘wow look at that distri-

bution change’ but we have proned

patients where they only open up on

one side, for example. Even after pron-

ing their left side, for example, is still

not open. This is a patient who may be

more complicated, you may prone

them because it’s the right thing to do

in terms of the evidence but in terms

of their lack of response it may be

reflected in what you see visually. The

patient I showed in the example

responded nicely and we were able to

reduce FIO2by 0.2 over the next couple

of hours. In some patients we are not

able to reduce any settings, they’re still

relatively hypoxemic but you can see

with EIT that they haven’t opened up

the same and some patients take way

longer. This example was very quick,

there’s another example we saw with

Tai [Pham] where within an hour aera-

tion was completely redistributed. I’ve

seen patients where it doesn’t happen

until 16 hours of proning and you

don’t see volume distribution that

impressive.

Pham: The patient he mentioned

was even more impressive than that,

because the patient showed in your

presentation, aeration was redistrib-

uted, so you had loss of ventilation in

the ventral region and gain in the dor-

sal region. But there are some patients

who only gain when they are proned,

which means you have reopened lung

homogeneously aerated as opposed to

separation between aerated and non-

aerated lung.

Scott: I have a question about EIT,

as I don’t have any experience with it.

Is there any risk of skin breakdown

from the band when you are placing a

patient in the prone position?

Piraino: The manufacturer recom-

mends you take it off every 24 hours.

We did have a learning experience

with proning. One of the issues with

proning is that the belt we use has 4

grommets you push through a hole so,

depending upon how tight the band is,

you may have more or less leftover

holes to attach the belt, and those holes

when you prone somebody, if you

don’t put something underneath, what

can happen is they can develop blisters



in the area or the holes. The blisters

subside and there was no evidence of

tissue injury, but we learned from our

experience and we now ensure that we

put gauze or padding under there and

haven’t had that issue since. We gener-

ally try to take it off every 24 hours

just for skin protection.

Scott: Can you move the location of

the EIT slightly up and down to miti-

gate this issue?

Piraino: There are data looking at

the various positions from armpit (as

high as you can go) to diaphragm.1

They found the optimal position is

around the 4th or 5th intercostal. So, if

you are between the 4th and 5th inter-

costal and you moved it up or down

one belt-width, when you look at the

article there isn’t a huge change when

you move one step up or down. If you

went from the maximum to lowest

position you’d see a huge change in

what the image looks like, but in the

areas above and below to my eye

were very similar in their examples.

Clinically we sometimes can’t get it to

stay at 4 or 5. In female patients for

example, it goes over breast tissue and

may slide up slightly. If you can move

it, even move it one belt-width, the

recommendation for taking it off, is

more related to its position on the

back. You may move it during nursing

care, but you don’t know that it’s stay-

ing the same place after moving back

and then repositioning. I think the idea

is to remove it, assess the skin, provide

any skin care and if there are no issues

it could be reapplied.

Schmidt: I’d like to frame what

sounds like an impossible situation,

but I’m hoping somebody has clever

ideas. It relates to ARDS and the set-

ting of PEEP. Depending on the tool

we’re using to look at PEEP level we

can prioritize recruitment or over-

distension or compliance or dead

space or the homogeneity of ventila-

tion and as you’ve shown in some of

your diagrams these don’t always line

up. I think even the figures that get

published usually the investigator

chooses the perfect example of where

things did or didn’t line up. In light of

depending on what lens we’re using

we see different things, how can we

make progress in determining how to

customize PEEP for the individual

patient?

Piraino: I think using the tools we

have is good. You bring up the pri-

mary point as to why nobody can

agree on how to set PEEP. Someone

will say, they use dead space fraction

(VD/VT) because they’ve had good

results overall and that’s their pre-

ferred method. If you look at the litera-

ture you may get a similar response

using respiratory system compliance,

which you’re right, in many patients

they don’t align and this adds to the

complexity of what is ARDS. PEEP ti-

tration may go well for one patient

using VD/VT, but if they have a pulmo-

nary embolism, it may not be very val-

uable. There are a number of factors

that can affect the utility of each tool

you can use. I don’t know that any-

body will actually agree on the best

way to set it.

Goligher: I think Greg’s [Schmidt]

question and the point behind it is very

well taken. To me, what we need to do

is to establish what aspect of this prob-

lem has the greatest impact on lung

injury. Is avoiding derecruitment more

important than avoiding over-disten-

sion? Personally my read of the litera-

ture is that over-distension is the major

driver of lung injury and that’s really

what we should try to minimize as

much as possible. What’s compelling

about using EIT at the bedside is there

often does seem to be a PEEP level at

which you can substantially minimize

over-distension while also achieving

80–90% of the recruitment that can be

attained – you can actually visualize

that. We’re running a trial now in

Toronto that uses an EIT titration

procedure to identify some version of

optimal PEEP and then randomize

subjects to that or to PEEP based on

the EXPRESS trial. We’re using bio-

logical measures of lung injury as a

very short-term surrogate for how the

lung responds to different levels of

PEEP. To clarify this issue we need to

randomize subjects to the PEEP level

at which we can see that recruitment is

relatively maximized versus the PEEP

level at which over-distension is rela-

tively minimized and progressively

work out which of these mechanisms

really drive lung injury. A lot of the

PEEP trials to date have been predi-

cated on the notion that atelectrauma

is a really important mechanistic

driver of lung injury. Personally, I’m

skeptical of that hypothesis.

Schmidt: I like that response and it

runs counter to the implication of the

Franchineau paper2 on ECMO subjects

and best PEEP based on balancing der-

ecruitment and over-distention as if

those are equivalent insults. It may be

that they are, and it visually is attrac-

tive to see the minimization of both,

but as you suggest these may not be

equivalent insults so it may be a mis-

leading idea to think of best being the

balancing of two things that may be

different.

Piraino: The other challenge that

will be facing EIT moving forward is

which method of PEEP selection you

use with EIT. There’s the Costa

approach,3 which I find interesting, but

my personal preference at the bedside is

simply balancing the distribution of

ventilation in all regions as much as

possible whether it’s a ventral-dorsal

balance, or if I’m just using the regions

that are important like the left and right.

That’s something that can be done in

real time. The analysis of the Costa

approach is that you push a button and

you can go back and analyze the data

that’s just been captured and trending

and most of the time I get a very similar

result as to where I should set the PEEP



within a couple cm H2O. But it’s a

very simplistic approach, but you

bring up a good point there have

patients where we’ve set the PEEP

not at that point but slightly above or

slightly below based on other param-

eters like what is their driving pres-

sure, what is their Pplat. I think using

just one method will be limited,

using many can be limited when they

don’t agree, but when you have

things like Pplat that we know

nobody’s arguing that the higher the

Pplat the more injurious it could pos-

sibly be. But using all values in con-

junction with what you’re seeing has

guided me. If the Costa approach

suggests a PEEP setting, but if my

Pplat is 35 and I decrease PEEP 2 cm

H2O and Pplat is now 30, I’ll likely

keep it there.

Goligher: You gave a nice presenta-

tion on the concept of strain and you

employed a helpful teaching approach

that I hope you highlight in your paper

because it’s a very difficult concept to

explain to trainees. I think it’s really

important. One of the issues that needs

to be better understood in enhancing

our thinking on PEEP is the relative

importance of global strain which is

tidal volume plus an increase in

end-expiratory lung volume vs dynamic

tidal strain, and which of those is more

important driver of lung injury. I’d be

interested to know what everybody

else here thinks, in the literature it

seems that the dynamic strain is prob-

ably far more important than global

PEEP or static strain. The interesting

twist or complexity being that the

more you lower the tidal volume the

less recruiting the lung impacts on

dynamic strain. And so even this

issue of how much should you opti-

mize the PEEP, increase it to recruit

the lung at the cost of increasing

global strain, probably depends on

how low you’re going with your tidal

volume. I think that’s an aspect of

mechanics that hasn’t been well

appreciated – the influence of tidal

volume on the optimal balance of

dynamic and global strain.

Piraino: Again, to add even more

complexity, the local strain repre-

sented by pendelluft.5 You’ve lowered

tidal volume you may think you’ve

lowered tidal global strain but in fact it

may be dependent on the patient’s

interactions with the ventilator. You

may have a result showing biomarkers

that are increasing which doesn’t

make sense if you’re thinking that

you’ve lowered global strain but it

could just be localized based on that.

REFERENCES

1. Karsten J, Stueber T, Voigt N, Teschner E,

Heinze H. Influence of different electrode

belt positions on electrical impedance tomog-

raphy imaging of regional ventilation: a pro-

spective observational study. Crit Care

2016;20:3.

2. Franchineau G, Brechot N, Lebreton G,

Hekimian G, Nieszkowska A, Trouillet J-L,

et al. Bedside contribution of electrical imped-

ance tomography to setting positive end-

expiratory pressure for extracorporeal

membrane oxygenation-treated patients

with severe acute respiratory distress syn-

drome. Am J Respir Crit Care Med 2017;

196(4):447-457.

3. Franchineau G, Brechot N, Lebreton G,

Hekimian G, Nieszkowska A, Trouillet JL,

et al. Bedside contribution of electrical im-

pedance tomography to setting positive end-

expiratorypressure for extracorporeal mem-

brane oxygenation-treated patients with

severe acute respiratory distress syndrome.

Am J Respir Crit Care Med 2017;196(4):

447-457.

4. Costa EL, Borges JB, Melo A, Suarez-

Sipmann F, Toufen C Jr, Bohm SH, Amato

MB. Bedside estimation of recruitable alveo-

lar collapse and hyperdistension by electrical

impedance tomography. Intensive Care Med

2009;35(6):1132-1137.

5. Yoshida T, Torsani V, Gomes S, De Santis

RR, Beraldo MA, Costa EL, et al. Spon-

taneous effort causes occult pendelluft during

mechanical ventilation. Am J Respir Crit

Care Med 2013;188(12):1420-1427.

This article is approved for Continuing Respiratory Care Educationcredit. For information and to obtain your CRCE

(free to AARC members) visitwww.rcjournal.com



lung volume measurement andventilation distribution during

Documents